Role of ICOS-B7h Regulatory Costimulatory Pathway in Fetomaternal Tolerance The fetus has paternal antigens which can evoke strong allogeneic T cell responses. However, in general the semi-allogeneic fetus is not usually rejected indicating the existence of active tolerance mechanisms that prevent rejection. It has been demonstrated by several investigators that apoptosis of maternal leukocytes, which could get activated in response to fetal antigens, plays a role in maternal tolerance to an allogeneic fetus. Negative signals delivered to activated T cells by regulatory T cell pathways also act as natural inhibitors for effector T cell expansion. Recent studies from our group has demonstrated that negative T cell costimulatory pathway PD-1-PDL1 is critical for maintaining fetomaternal tolerance. ICOS-B7h is another costimulatory molecule with immunoregulatory properties. We have been able to show the expression of B7h in the placentae of mice. In order to study if blockade or signaling through this pathway alters pregnancy outcomes and hence suggest a role in tolerance, we utilized an established model of allogeneic pregnancy in which CBA females are mated with C57BL/6 males. The spontaneous rate of resorption in this model is approximately 20%. Our preliminary results show that ICOS/B7h is involved in fetomaternal tolerance as in vivo blockade of this pathway resulted in increase in rate of spontaneous resorption (from 20% in normal matings to 53% in the anti- B7h mAb treated group). This effect was seen only in allogeneic but not in syngeneic concepti. We will extend our studies to investigate the functions and mechanisms of the ICOS-B7h pathway in regulating the process of fetal allograft acceptance or rejection. We will further delineate the in vivo mechanisms involved in ICOS-B7h pathway induced fetomaternal tolerance utilizing ICOS and B7h specific blocking antibodies as well as gene deficient mice. The fetus represents a foreign entity to the maternal immune system, yet this "natural" allograft is not normally rejected. Fifty years ago, it was proposed by Medawar (Medawar, P.B. 1953. Symp. Soc. Exp. Biol. 7:320-338) that immunological tolerance should be present during pregnancy to protect against an aggressive maternal alloimmune response directed at the paternal antigens expressed by the fetus. Recurrent pregnancy loss affects 1% to 3% of all couples, and about half of these cases have no identifiable cause. Furthermore, a number of studies associate some pregnancy complications with abnormal maternal immune responses. Recent studies from our group has demonstrated that negative T cell costimulatory pathway PD-1-PDL1 is critical for maintaining fetomaternal tolerance. ICOS-B7h is another costimulatory molecule with immunoregulatory properties. We have been able to show the expression of B7h in the placentae of mice. We will study the role of ICOS-B7h, costimulatory molecule with immunoregulatory properties in fetomaternal tolerance utilizing ICOS and B7h specific blocking antibodies as well as gene deficient mice. Understanding the complex mechanisms of fetomaternal tolerance has important implications for developing novel strategies to prevent or reduce spontaneous abortion in at-risk populations in particular.